This invention is directed to a multi-stage process for hydroprocessing vacuum gas oils and other feeds. In addition to gases and middle distillates, this process can produce unconverted oil which is suitable for use as base oil feed for white oils, Group III oils, and low BMCI (Bureau of Mines Correlation Index) ethylene plant feed.
Suitable base stocks for Group III oils have traditionally been produced in a variety of ways. U.S. Pat. No. 6,136,181 (Ziemer) discloses a process for hydrofinishing and hydrocracking feeds containing sulfur and nitrogen to produce base stocks suitable for use in preparation of Group III oils and white oils. A catalyst comprising a platinum-palladium alloy is employed.
U.S. Pat. No. 6,099,719 (Cody et al.) discloses a process for the preparation of lube oil basestocks suitable for Group III oils. A lube oil feedstock is subjected to solvent extraction and solvent dewaxing prior to a two-step hydroconversion process, which is followed by hydrofinishing and dewaxing steps.
U.S. Pat. No. 5,580,442 (Kwon et al.) employs recycle of unconverted oil to produce high quality lube base oil. VGO is produced by vacuum distillation, then hydrotreated. The hydrotreated VGO is then hydrocracked and light hydrocarbons, along with light oil products, are removed. A portion of the unconverted oil is fed to a second vacuum distillation unit. Material not converted to products in the vacuum distillation unit is recycled to the hydrocracker.
Another approach to obtaining Group III basestocks involves two-stage hydroprocessing, in which the effluent from a first stage operated at low pressure is mixed with second stage effluent. The resultant mixture is sent to the fractionation section for product recovery at low pressure. Often a bleed stream from the unconverted oil is taken for feed to the downstream units (such as Group III base oil production or ethylene cracking). The quality of this unconverted oil is not sufficiently high, without further processing to be used as Group III base oil feed or low Bureau of Mines Correlation Index ethylene plant feed.
In the configuration of this invention, the feed to the second stage is a mixture of first and second stage unconverted oil. The first stage is operated at high pressure and the second stage is operated at a lower pressure. The feed to the second stage is high quality unconverted oil, and may be used as feed for Group III base oil production, ethylene plant feed, white oil production, etc.
The invention is summarized below:
1. A method for hydroprocessing a hydrocarbon feedstock which produces a stream of unconverted oil of sufficient quality for use as a base oil feed for the production of Group III oils, white oils, and low BMCI ethylene plant feed, said method employing multiple reaction zones within a single reaction loop, comprising the following steps:
(a) passing a hydrocarbonaceous feedstock to a first hydroprocessing zone, the hydroprocessing zone having one or more beds containing hydroprocessing catalyst, the hydroprocessing zone being maintained at hydroprocessing conditions, including a pressure in the range from 1200 to 2500 psig, wherein the feedstock is contacted with catalyst and hydrogen;
(b) passing the effluent of step (a) directly to a hot high pressure stripper, wherein the effluent is contacted with a hydrogen-rich stripping gas to produce a vapor stream comprising hydrogen, hydrocarbonaceous compounds boiling at a temperature below the boiling range of the hydrocarbonaceous feedstock, hydrogen sulfide, ammonia, and a bottoms stream comprising hydrocarbonaceous compounds boiling in approximately the same range of said hydrocarbonaceous feedstock along with a portion of the hydrocarbonaceous compounds boiling in the diesel boiling range;
(c) passing the overhead vapor stream from the hydrogen stripper of step (b) to a first cold high pressure separator where hydrogen, hydrogen sulfide and light hydrocarbonaceous gases are removed overhead and a liquid stream comprising naphtha, middle distillates and unconverted oil is passed to fractionation, thereby removing most of the ammonia and some of the hydrogen sulfide (as ammonium bi-sulfide in the sour water stream as it leaves the cold high-pressure separator);
(d) combining the liquid stream from the hydrogen stripper of step (b) with a portion of the unconverted oil of the fractionation step of step (c) and passing the combined stream to a bed of hydroprocessing catalyst in a second reactor zone, wherein the liquid is contacted under hydroprocessing conditions with the catalyst, in the presence of hydrogen, and under a pressure in the range from 1500 to 2500 psig;
(e) passing the overhead from the cold high pressure separator of step (d) to an amine absorber, where hydrogen sulfide is removed before hydrogen is compressed and recycled to hydroprocessing vessels within the loop;
(f) passing the effluent of step (d), after cooling, to a second cold high pressure separator where hydrogen, hydrogen sulfide and light hydrocarbonaceous gases are removed overhead and a liquid stream comprising naphtha, middle distillates and unconverted oil is passed to fractionation, thereby removing most of the ammonia and some of the hydrogen sulfide (as ammonium bi-sulfide in the sour water stream as it leaves the second cold high-pressure separator);
(g) passing the vapor stream from step (f) after further cooling and separation of condensate, to the recycle gas hydrogen compressor;
(h) passing the compressed hydrogen from the recycle gas compressor to the primary reactor loop; and
(i) passing at least a portion of the unconverted oil from the fractionator of steps (c) and (f) to facilities for the preparation of Group III oil, white oil, or BMCI ethylene feed.
The instant invention provides reduced capital investment and operating costs, as compared with the traditional two stage hydroprocessing scheme.